Page 362 - Pipelines and Risers

Pipeline Inspection, Maintenance and Repair 333

mm. The smallest defect to be detected and properly sized has a width equal to the sensor
spacing and a length equal to about three times the axial sampling distance.

Within the intelligent pigging industry, a distinction is made between low resolution and high
resolution MFL pigs refemng to the quality of measurement. However, it should be noted that
a proper definition on low and high resolution is non-existent. Therefore the fact that an MFL
pig is called high resolution does not guarantee a good performance.

Many MFL pigs contain additional sensors to discriminate between internal and external
defects and to get a measure of wall thickness changes. InternaVexternal discrimination is
done by means of sensors that are only sensitive to internal defects. Most contractors apply
weak magnets combined with a magnetic field sensor placed in a second sensor ring outside
the magnetic yoke that measure the decrease in magnetic field when the lift off distance of the
magnet to the pipe wall increases by internal metal loss defects. Some contractors make use of
eddy current proximity probes that may be placed within the magnetic yokes.

A measure of the wall thickness is obtained by measuring the axial background magnetic field
by means of Hall effect sensors. The axial background magnetic field is related to pipe wall
magnetization and thus pipe wall thickness.

Data Analysis
MFL pigs record a large amount of data that needs to be analyzed. Most contractors have
developed software that automatically analyze the data and detect the relevant features.
However, manual analysis and data checks are still necessary to obtain the most accurate
defect data.

The relation between Mm, signals and defect dimensions is indirect and non linear.
Consequently good data analysis algorithms are of importance. Defect length can be
accurately determined from the start and end of the MFL signals. Defect width can be
determined with limited accuracy from the circumferential signal distribution as measured by
adjacent sensors. Defect depth is related to the integrated signal amplitudes but corrections
have to be made for defect length and length / width aspect ratios. For defects with a length
above 3t (t = wall thickness) or 30 mm, this relation tends to become linear. The relationship
between metal loss defect depth and MFL signals becomes more non linear and length
dependent below a defect length of 3t or 30 mm for which reason defect sizing accuracy will
be of lesser quality.

Capabilities and Limitations
Defect detectability levels are highly dependent on the magnetization level in the pipe wall,
the MFL noise as generated by the pipe and the geometry metal loss defect

The pipe material make influence magnetic noise levels. In particular seamless pipe creates a
high magnetic noise level whilst on the other hand the ERW manufacturing process gives